1. Field of the Invention
The present invention relates to a high-voltage transformer protection circuit, and particularly to the improvement of a current shut-off function which is provided against the occurrence of abnormalities of a high-voltage transformer.
2. Description of the Prior Art
For the generation of a high voltage, there have been widely used high-voltage transformers such as a flyback transformer. For example, in order to supply a high voltage to the high-voltage electrode of a CRT (cathode ray tube) of a television receiver or the like, the above-mentioned high-voltage transformer is necessary. Because of increasing demands of CRT's as a display device along with the progress of computer equipment, the importance of high-voltage transformers is also rising steadily.
An example of circuits using such high-voltage transformers will be explained on FIG. 3.
This circuit is a general circuit for the deflector used in a CRT, and it comprises a horizontal deflection output circuit 30 and a high-voltage transformer 10. The high-voltage transformer 10 supplies its output to a CRT 32.
The horizontal deflection output circuit 30 includes a horizontal output transistor T.sub.R1, a damper diode D.sub.1, a resonance capacitor C.sub.1, a horizontal deflection coil L.sub.H, and an S-shape correction capacitor C.sub.S. The horizontal output transistor T.sub.R1 receives a voltage pulse signal sent from the horizontal oscillation circuit (not shown) and implements the prescribed switching operation. The horizontal output transistor T.sub.R1 and damper diode D.sub.1 operate in unison to supply a saw-tooth current to the horizontal deflection coil L.sub.H. In response to the current, the horizontal deflection coil L.sub.H and resonance capacitor C.sub.1 operate resonantly to generate a flyback pulse, which is applied to the high-voltage transformer 10.
The high-voltage transformer 10 has its primary winding N.sub.1 connected at one end to the common terminal of the cathode of the damper diode D.sub.1, the horizontal deflection coil L.sub.H and the resonance capacitor C.sub.1, and connected at another end T.sub.1 to the input power source E.sub.B. The high-voltage transformer 10 has its secondary winding N.sub.H connected at the end of high-voltage side to the high-voltage electrode (anode) of the CRT (cathode ray tube) 32 through a high-voltage rectifying diode D.sub.H.
Connected in the secondary circuit, which extends from the output terminal of the secondary winding N.sub.H to the anode 34 of the CRT 32, are a focus pack F.sub.P and a smoothing capacitor C.sub.H. The focus pack F.sub.P is made up of a serial connection of resistors R.sub.7, R.sub.8, R.sub.V1 and R.sub.V2, and it serves to supply a specified voltage to the screen electrode and focus electrode of the CRT 32.
In the foregoing circuit arrangement, the high-voltage transformer 10 steps up the voltage of the flyback pulse provided by the horizontal deflection output circuit 30, the high-voltage rectifying diode D.sub.H rectifies the pulse voltage, and the resulting high voltage E.sub.H is applied to the anode 34. The smoothing capacitor C.sub.H is used to smooth the high voltage E.sub.H.
If the high-voltage transformer 10 has its secondary winding N.sub.H short-circuited by some reason, and increased current can possibly cause heat generation or smoke eruption.
On this account, there have been devised heat/smoke preventive means also in the conventional equipment. The conventional protection circuit will be explained on FIG. 4.
This example is intended to detect an excessive current in the power source E.sub.B in FIG. 3 thereby to limit the current supply. The circuit bases its operation on the increase in the primary current I.sub.B at the occurrence of a short-circuit on the secondary winding N.sub.H.
In this example, the power source E.sub.B is arranged as follows. An input end P is a power input plug, which is plugged to the ordinary power outlet of a.c. 100 volts. The input end P is led to a rectifying circuit 42 made up of diodes D.sub.21, D.sub.22, D.sub.23 and D.sub.24 in the bridge configuration. The rectifying circuit 42 is connected on its output side with a voltage stabilizing circuit 44 so that a specified d.c. power is supplied to the high-voltage transformer 10. Namely, the output end 46 is connected to one end T.sub.1 of the primary winding N.sub.1 in FIG. 2. The capacitor C.sub.21 is used to smooth the voltage.
The voltage stabilizing circuit 44 consists of a transistor T.sub.R21, a resistor R.sub.20 and a control circuit 48. The control circuit 48 receives the output voltage of the voltage stabilizing circuit 44, and it controls the base voltage of the transistor T.sub.R21 so that the voltage maintain the specified level. Accordingly, the voltage stabilizing circuit 44 has its output voltage controlled to a virtually constant level, and consequently the voltage E.sub.B at the output end 46 is also virtually constant. A capacitor C.sub.32 is used to smooth the voltage, particularly to improve the transitional response.
This example further includes an abnormal current detecting circuit 50 provided between the voltage stabilizing circuit 44 and output end 46 thereby to monitor the current I.sub.B flowing in it. The abnormal current detecting circuit 50 detects the voltage across a resistor R.sub.22 which is inserted between the voltage stabilizing circuit 44 and output end 46, and it turns on a transistor T.sub.R22 when the resistor voltage exceeds a predetermined level thereby to detect an abnormal current.
The resistor R.sub.22 is connected at its one end on the upstream side with the emitter of the transistor T.sub.R22, and at its another end on the downstream side with the base of the transistor T.sub.R22 through a resistor R.sub.23. A capacitor C.sub.22 is used to absorb a short-term variation of the current I.sub.B.
In the conventional high-voltage transformer protection circuit arranged as described above, when the current I.sub.B increases, causing a voltage difference across the resistor R.sub.22 to exceed the specified value, the base-emitter voltage of the transistor T.sub.R22 exceeds the specified value, and the transistor T.sub.R22 becomes conductive. The transistor T.sub.R22 has its collector grounded through resistors R.sub.24 and R.sub.25. Accordingly, when a current flows in the conductive transistor T.sub.R22, the voltage at the node of the resistor R.sub.24 and R.sub.25 will rise by the amount of the voltage drop across the resistor R.sub.25. A capacitor C.sub.24 is used to remove the a.c. component at the node of R.sub.24 and R.sub.25.
The node of the R.sub.24 and R.sub.25 is connected to the base of a transistor T.sub.R23, which has its collector connected to one input end of a photocoupler 52 and has its emitter grounded. The photocoupler 52 has another input end connected through a resistor R.sub.26 to a power source E.sub.C21. Accordingly, when the transistor T.sub.R23 turns on, a light emitting diode 52a connected at the input end of the photocoupler 52 is activated. A transistor 52b on the light-sensitive side of the photocoupler 52 has its base grounded through a resistor R.sub.27, its collector connected to a power source E.sub.C22, and its emitter connected to a horizontal oscillation circuit 54.
When the light emitting diode 52A of the photocoupler 52 emits the light, causing the transistor 52b to turn on, an oscillation halt signal is entered to the horizontal oscillation circuit 54. Accordingly, the horizontal oscillation circuit 54 ceases oscillating in response to the reception of the oscillation halt signal. The oscillation pulse from the horizontal oscillation circuit is entered to the input terminal of the drive circuit 30 in FIG. 2, and therefore the subsidence of the pulse causes the drive circuit 30 to halt its operation. Consequently, the high-voltage transformer 10 does not generate a flyback pulse, whereby the high-voltage transformer 10 can be prevented from heat generation or smoke eruption.
The role of the photocoupler 52 is to provide the insulation (a.c. isolation) between the charging section (a.c. side) and the non-charging section which drives the horizontal oscillation circuit 54.
The conventional high-voltage transformer protection circuit has a large number of component parts and impose a high manufacturing cost, and yet suffers a low reliability. The need of a.c. isolation further increases the number of component parts.
Moreover, according to the conventional high-voltage transformer protection circuit, when the photocoupler 52 operates once to deactivate the horizontal oscillation circuit 54, if the main power supply is turned off the then turned on again, the current I.sub.B is not prevented from flowing during a period until the photocoupler 52 operates again.
By providing the high-voltage transformer 10 with a thermal fuse which a blown at a specified high temperature, the current to the high-voltage transformer 10 can be shut off. However, the high-voltage transformer has a drive frequency as high as 15.75 kHz to 130 kHz, and therefore the disposition of such a bulky component as a thermal fuse lowers the electromagnetic coupling, resulting in a deteriorated fundamental performance of the high-voltage transformer.